Metallurgical-plant converters, utilized to perform refining operations on metallurigical melts, generally emit large volumes of exhaust gas which is made up of fumes, particulates, reaction products and entrained gases.
Before any part of these gases can be released into the environment for health and safety reasons, particulates must be removed therefrom and the removal of such particulates may be desirable on economical grounds as well to rectify variable components of the entrained solids.
The most frequently used gas cleaning system for the exhaust or waste gases of a converter, employs a scrubber generally having a cooling and saturating stage ahead of a scrubbing stage.
In the first stage, the exhaust gases can be cooled to a temperature of 60.degree.-80.degree. C. During the operation, the gases undergo a pressure drop of 200 to 400 mm water column. A pressure drop of 1200 to 1400 mm water column is required for the second stage. Because the dust is very fine, such scrubbers have been found to be effective only to remove dust in amounts above about 100 mg/m.sup.3 STP, the gas containing a residual solids concentration of this magnitude.
Operation of scrubbers is energy intensive at least in part because of the substantial pressure drops, because of the need for blower power to develop such pressure drops.
Furthermore, because of the comparatively high residual dust content, the gases generally cannot be used directly for other purposes without further purification e.g. in a bag filter, and certainly cannot under existing environmental standards be released in whole or in part to the atmosphere.
In more modern plants, dry-process electrostatic precipitators have generally superceded scrubbers, because they can be operated with reduced pressure drops, residual dust concentrations and hence greater economy.
Further, dry process electrostatic precipitators cannot readily be installed in existing metallurgical plants to replace scrubbers, especially because they are not compatible with the preceding stages and because long term shutdown of the plant would have to be contemplated along with considerable redesign. In some gases, the space requirements for dry-process scrubbers will not admit of such replacement in any event.
Obviously an attack on the problem may be made by providing wet-process electrostastic precipitators in place of the energy consuming scrubbing or second stage of the conventional scrubbing process hitherto used. This could avoid a prolonged shutdown because the entire wet-process precipitator could be fabricated off site, transported to the plant and installed with a minimum of down time while the steel works continues its production and the gas flow paths connected to the new unit.
Further, while considerable interest has been expressed in this possibility, as far as I am aware, prior to the contribution described below, there has been no significant success in the use of wet-process electrostatic precipitators in the treatment of exhaust gases from metallurigical plant converters and especially from steel making converters.
Apparently, if there have been earlier efforts to utilize electrostatic precipitators for converters, these have proved to be unsuccessful because of difficulties engendered by the composition of the converter waste gas. Converter waste gases are notoriously explosive and combustible so that in the handling of them, there is always the risk of detonation not only in the treating unit itself, but in the entire system.
Electrostatic precipitators have been provided in systems sensitive to explosion with pressure resistant housings or even housings with portions which can be readily displaced to release the energy of explosions but, as far as I am aware, these have not been utilized with great success for converter gases if at all.
Other problems which may have been encountered heretofore and have contributed to the difficulties may derive from the need for extremely large flow cross sections because of the need to treat extremely large volumetric flows which are generated for brief periods of time at high velocities and the difficulties which have hithereto been encountered in treating gases at the available velocities of 1 to 1.8 meters per second. Theoretically, using conventional designs and the requirements for processing converter gases, it has been calculated that electrode heights of 10 meters or more may be required. This of course creates a problem with respect to the stability of the electrode system and introduces the need for stabilizing or regidifying structures.
It would be desirable to utilize electrode plates with a height of 3 to 5 meters at a maximum, but such plates utilizing conventional precipitator designs cannot effectively be rinsed under the conditions contemplated for the treatment of converter gases.
Exhaust gases from a converter are usually saturated before entering the collected fields of the precipitator so that condensate as well as moist dust accumulates on the collecting electrodes.
Generally speaking exhaust gases from a converter are available only intermittently so that adequate time between treatment intervals is available for rinsing and hence continuous rinsing is not necessary. During the blowing period, however, the gas is supplied at such rates that rinsing must in any event be interrupted so that the electrostatic precipitator can be operated on the highest possible voltage. Obviously voltage control fails if high voltage levels are applied concurrently with rinsing.
When the dust collected in a moist state or as a sludge is to be removed by liquid sprayed from nozzles disposed outside the electrical field, the jets of spray must be sufficiently fine to allow a substantially uniform distribution over the plates, but each individual streamlet must impinge with an energy sufficient to scrub the plate free from the collected dust or sludge.
Experience has shown that the two requirements of uniform fine spray and high voltage streamlets cannot be met at the same time unless the spraying nozzle is brought sufficiently close to the surface that the kinetic energy of the streamlet will not significantly drop after leaving the spraying jet. For practical reasons, therefore, the height of thes collecting electrode plate must be kept between 3 and 5 meters since greater heights will interfere with the spray requirements and require the stabilizing structures and smaller heights are impractical.
Furthermore present environmental and economical requirements mandate that the precipitator purify the gases to a residual solids content no greater than 10 mg/m.sup.3 STP at a much lower pressure drop than previously utilized for scrubbers.